RNA interference (RNAi) in animals and basal eukaryotes, quelling in fungi, and posttranscriptional gene silencing (PTGS) in plants are examples of a broad family of phenomena collectively called RNA silencing (Kooter et al. 1999; Li and Ding 2001; Matzke et al. 2001; Vaucheret et al. 2001; Waterhouse et al. 2001; Hannon 2002; Plasterk 2002). The unifying features of RNA silencing phenomena are the production of small (21-26 nt) RNAs that act as specificity determinants for down-regulating gene expression (Hamilton and Baulcombe 1999; Hammond et al. 2000; Parrish et al. 2000; Zamore et al. 2000; Djikeng et al. 2001; Parrish and Fire 2001; Tijsterman et al. 2002) and the requirement for one or more members of the Argonaute family of proteins (or PPD proteins, named for their characteristic PAZ and Piwi domains) (Tabara et al. 1999; Fagard et al. 2000; Hammond et al. 2001; Hutvágner and Zamore 2002; Kennerdell et al. 2002; Martinez et al. 2002a; Pal-Bhadra et al. 2002; Williams and Rubin 2002).
Small RNAs are generated in animals by members of the Dicer family of double-stranded RNA (dsRNA)-specific endonucleases (Bernstein et al. 2001; Billy et al. 2001; Grishok et al. 2001; Ketting et al. 2001). Dicer family members are large, multidomain proteins that contain putative RNA helicase, PAZ, two tandem ribonuclease III (RNase III), and one or two dsRNA-binding domains. The tandem RNase III domains are believed to mediate endonucleolytic cleavage of dsRNA into small interfering RNAs (siRNAs), the mediators of RNAi. In Drosophila and mammals, siRNAs, together with one or more Argonaute proteins, form a protein-RNA complex, the RNA-induced silencing complex (RISC), which mediates the cleavage of target RNAs at sequences with extensive complementarity to the siRNA (Hammond et al. 2000, 2001; Zamore et al. 2000; Elbashir et al. 2001a, b, c; Nykänen et al. 2001; Hutvágner and Zamore 2002; Martinez et al. 2002a).
In addition to Dicer and Argonaute proteins, RNA-dependent RNA polymerase (RdRP) genes are required for RNA silencing in Caenorhabditis elegans (Smardon et al. 2000; Sijen et al. 2001), Neurospora crassa (Cogoni and Macino 1999), and Dictyostelium discoideum (Martens et al. 2002), but likely not for RNAi in Drosophila or mammals (Celotto and Graveley 2002; Chiu and Rana 2002; Holen et al. 2002; Martinez et al. 2002b; Schwarz et al. 2002; Roignant et al. 2003). In plants, PTGS initiated by transgenes that overexpress an endogenous mRNA also requires a putative RdRP, SGS2 (SDE1; Dalmay et al. 2000; Mourrain et al. 2000), although transgenes designed to generate dsRNA bypass this requirement (Beclin et al. 2002). Similarly, silencing induced by viruses replicating through a dsRNA intermediate (virus-induced gene silencing, VIGS) does not require SGS2 (Dalmay et al. 2000).
Dicer in animals and CARPEL FACTORY (CAF, a Dicer homolog) in plants also generate microRNAs (miRNAs), 20-24-nt, single-stranded noncoding RNAs thought to regulate endogenous mRNA expression (Lee et al. 1993; Reinhart et al. 2000, 2002; Grishok et al. 2001; Hutvágner et al. 2001; Ketting et al. 2001; Lagos-Quintana et al. 2001, 2002; Lau et al. 2001; Lee and Ambros 2001; Mourelatos et al. 2002; Park et al. 2002). miRNAs are produced by Dicer cleavage of stem-loop precursor RNA transcripts (pre-miRNAs); the miRNA can reside on either the 5′ or 3′ side of the double-stranded stem (Lee et al. 1993; Pasquinelli et al. 2000; Lagos-Quintana et al. 2001; Lau et al. 2001; Lee and Ambros 2001). In animals, pre-miRNAs are transcribed as longer primary transcripts (pri-miRNAs) that are processed in the nucleus into compact, folded structures (pre-miRNAs), then exported to the cytoplasm, where they are cleaved by Dicer to yield mature miRNAs (Lee et al. 2002). Animal miRNAs are only partially complementary to their target mRNAs; this partial complementarity has been proposed to cause miRNAs to repress translation of their targets, rather than direct target cleavage by the RNAi pathway (for review, see Ruvkun 2001; Hutvágner and Zamore 2002). Plant miRNAs have far greater complementarity to cellular mRNAs and have been proposed to mediate target RNA cleavage via an RNAi-like mechanism (Llave et al. 2002b; Rhoades et al. 2002).